Aedes aegypti and dengue

Dengue, a mosquito-borne viral disease, is endemic and prevalent in Singapore. It is also known as break-bone fever, which points to the severe muscle and joint pains that one can get when contracting dengue.1 Such is the prominence of this disease that it has become a household name in Singapore, and decades of public health messaging has educated the public well that the culprit of this disease is a “black-and-white mosquito”, the Aedes aegypti.2 Many will be surprised to know that this species has a non-native origin, given its ubiquitous nature and ability to thrive in Singapore’s urban niches. Weaving together history with scientific evidence, this story traces the challenges of Ae. aegypti vector control in Singapore and explores why dengue is still around despite decades of intensive vector control efforts.

A human-loving mosquito

Ae. aegypti is an anthropophilic species. Not only does it feed preferentially on humans,3 it almost exclusively breeds in specialised artificial container habitats such as flowerpots and bath tubs.4 They can breed in small amounts of water that are relatively low in organic matter and high in oxygen.5 Such adaptations have enabled this species to take easy advantage of stagnant water habitats.

The eggs of Ae. aegypti are laid just above the waterline of containers and are desiccant-resistant, which means they can survive without water for months at a time.6 Originating from the west Sahel region of Africa, this species has since spread around the globe. The egg form is likely how it was introduced into Singapore, through trade routes carrying artificial containers with accumulated water.

Though the first record of it in 1908 in Singapore was as Stegomyia fasciata,7 the description of the mosquito’s distinctive lyre-shaped white scales on its thorax means identification as Ae. aegypti is unmistakeable. Despite its medical significance importance and far-reaching impacts on public health now, the first detection of this species on the shores of Singapore was met without much alarm and was merely treated as a new insect record.

Outbreaks in Singapore

The increase in mosquito-borne diseases as a result of rapid urbanization happening in Singapore is not new. Since the 1900s, the island’s malaria cases were rampant because the malarial vectors (Anopheles species) thrive in disturbed forest edge habitats after forests were cleared for agriculture.8 Authorities dealt with the malaria problem swiftly, by eliminating breeding habitats such as seepages and ditches and tightening surveillance measures.9 Singapore was declared malaria-free by the World Health Organization in 1982.10 Despite Singapore’s best efforts, however, stemming dengue transmission has not been as successful, largely owing to the increase in urban habitats ideal for Ae. aegypti.

After World War II and the end of the Japanese occupation, citizens of Singapore focused on recovering their livelihoods, and many turned to hawker trades to earn income.11 Without proper sanitation and refuse systems, however, waste soon piled up and provided ideal conditions for the accumulation of stagnant water for mosquitoes to breed.12 The development of Singapore with the clearing of land to make way for buildings during this period marks the gradual transition of an increased availability of suitable urban habitats for Ae. aegypti.

While cases of dengue had been recorded in Singapore as early as 1901,13 it was not until 1960, with an outbreak of an urban and more severe form – the dengue haemorrhagic fever (DHF) – that more attention turned from fighting malaria to dengue.14 A year later, Dr. Albert Rudnick, an entomologist and virologist from the University of California, sampled mosquitoes from Singapore and established that the haemorrhagic form was transmitted by Ae. aegypti.15 By then, this non-native species had firmly established itself in Singapore’s growing urban landscape. Subsequently, Ae. aegypti facilitated many outbreaks of dengue and other mosquito-borne diseases in Singapore such as chikungunya in 2008,16 and more recently Zika in 2016,17 further highlighting its impact on epidemics in the urban areas.

The relentless chase

The first DHF outbreak in 1960 recorded 70 hospitalised cases18 and was a catalyst for authorities to set up the first governmental department dedicated to vector control, the Vector Control Unit (later the Vector Control Research Department – VCRD).19 Prior to this, the control of mosquitoes was seen as a public health responsibility under the Health department. This move placed vector control at the forefront of the government agenda and facilitated the set-up of an integrated system to tackle the growing dengue challenge. It was a nation-wide effort, involving regular surveillance and insecticide application as well as infrastructural changes such as the development of piping systems to phase out the need for stored water containers, which were–-and remain--a prime breeding habitat for Ae. aegypti.

While the use of insecticides was effective in killing mosquitoes, their widespread application and the fast generation times of the mosquito contributed to the development of insecticide resistance. To combat this, Singapore has adapted a novel technology, releasing millions of male Ae. aegypti infected with the insect bacteria Wolbachia. This control strategy is species-specific, and has been effective in bringing Ae. aegypti populations down in areas where the Wolbachia-infected males were released.

Following the success of the Wolbachia control method, there are concerns that other locally occurring and adaptable species may start to take over the ecological niche left by Ae. aegypti. The Asian Tiger mosquito (Aedes albopictus) is a widespread species in Singapore, occupying a wide range of vegetated to urban habitats. A “black-and-white” mosquito as well, it belongs to the same classification of subgenus Stegomyia as Ae. aegypti, and is a competent vector for dengue, chikungunya and Zika.21 Already it is a widespread species involved in multiple epidemics worldwide.22 This is a good reminder that Aedes mosquitoes and humans are part of the urban ecological fabric, and intentionally removing a species may not be as straightforward as it seems.

It takes a village for successful vector control

Ae. aegypti has been under intensive mosquito control in the past 60 years with noticeable decrease in its population size.23 Dengue cases are still on the rise however, with the top two highest cases in a year reported in the last three years (2020: 35,315 cases and 2022: 32,325 cases).24 This is despite the multitude of efforts and variety of vector control solutions in “chasing” down Ae. aegypti.

Inevitably, many of the breeding sites of Ae. aegypti occur indoors and mainly within residences which are personal spaces and largely inaccessible for surveillance. Recognising this early on in the fight against dengue, the authorities rolled out a variety of anti-mosquito campaigns to educate the public and encourage kampung spirit (gotong royong) to look out for one another.25

This includes broadcasting anti-mosquito messages on the television and radio, and a concise guide to “5-step mozzie wipeout” which identified common household areas where Ae. aegypti breed.26 The citizens were largely cooperative and receptive to the public health messaging, although interestingly, there were also reports of residents continuing to breed mosquito larvae as food for their fighting fishes,27 a popular pastime at that time, even with ongoing dengue outbreaks in the 1970s.28

To ensure that residents remain vigilant in eradicating Aedes breeding sites, the Disease-bearing Insects Act was passed in 1968 (now superseded by the Control of Vectors and Pesticides Act in 1998). The Act illegalises breeding of any mosquito species at home and enable public health officers to enter and examine home premises for mosquito breeding.29 Fines are also imposed as a deterrent, and penalties have gotten more stringent over the years, suggesting that a proportion of the population continues to be negligent.

Today, the rise in dengue cases is partially owed to the decades-old successful vector control strategies. Ironically, low numbers of Ae. aegypti in Singapore have resulted in a large pool of susceptible citizens (low herd immunity), which means that more people are likely to succumb to dengue when an outbreak happens. These sentiments have been echoed by various authorities throughout the years.

Conclusion

Despite its non-native status, Ae. aegypti has firmly established itself in Singapore’s urban ecosystem. It is part of the urban wildlife, much like cockroaches and otters. Singapore is widely acknowledged as a country with a world-class mosquito surveillance system and the persistence of Ae. aegypti and dengue is not for the lack of vector control efforts. The complex relationships and interdependency between humans and the local vector Aedes species mean that vector control and management of dengue transmission remains a challenge for the many years to come.

References

  1. “Illness in Singapore,” Straits Echo, July 5, 1918. ↩

  2. “THE CULPRIT,” New Paper, August 7, 1992. ↩

  3. Noah H Rose et al., “Dating the Origin and Spread of Specialization on Human Hosts in Aedes aegypti Mosquitoes,” eLife 12 (2023): e83524, https://doi.org/10.7554/eLife.83524. ↩

  4. World Health Organization, “Dengue Haemorrhagic Fever Control Programme in Singapore: A Case Study on the Successful Control of Aedes aegypti and Aedes albopictus Using Mainly Environmental Measures as a Part of Integrated Vector Control” (World Health Organization, 1986). ↩

  5. National Environment Agency, “Stop Dengue Now,” accessed April 5, 2023, https://www.nea.gov.sg/dengue-zika/stop-dengue-now; Noah H Rose et al., “Dating the Origin and Spread of Specialization on Human Hosts in Aedes aegypti Mosquitoes,” ELife 12 (2023): e83524, https://doi.org/10.7554/eLife.83524. ↩

  6. Steven A. Juliano et al., “Desiccation and Thermal Tolerance of Eggs and the Coexistence of Competing Mosquitoes,” Oecologia 130 (2001): 458–69, https://doi.org/10.1007/s004420100811. ↩

  7. G. F. Leicester, “The Culicidae of Malaya,” Studies from the Institute for Medical Research, Federated Malay States 3, no. 3 (1908): 18–261. ↩

  8. K. T. Goh, “Eradication of Malaria from Singapore,” Singapore Medical Journal 24, no. 5 (October 5, 1983): 255–68; Tyrone Ren Hao Tan and Huiqing Yeo, “Land Use Changes, Malaria Outbreaks, and Eradication Campaigns during the 1960s-1980s,” Singapore Ecological History Map, accessed April 5, 2023, https://nus.edu.sg/nuslibraries/dsprojects/singapore_ecological_history_map/mosquitos.html. ↩

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  10. Goh. ↩

  11. Victor Mathuraretnam Samuel Thevathasan, The Public Service 新加坡公共服务史 Interview with THEVATHASAN, Victor Mathuraretnam Samuel (Dr.) @ Victor Madura Ratnam, vol. Accession Number 002171/6, Reel 3, Civil Servants, 1945-1980, 1999, https://www.nas.gov.sg/archivesonline/oral_history_interviews/interview/002171. ↩

  12. Thevathasan, The Public Service 新加坡公共服务史 Interview with THEVATHASAN, Victor Mathuraretnam Samuel (Dr.) @ Victor Madura Ratnam. ↩

  13. Jayanthi Rajarethinam et al., “Dengue in Singapore from 2004 to 2016: Cyclical Epidemic Patterns Dominated by Serotypes 1 and 2,” The American Journal of Tropical Medicine and Hygiene 99, no. 1 (July 2018): 204, https://doi.org/10.4269/ajtmh.17-0819. ↩

  14. Sara Webb, “After Taming Malaria, Singapore Fights Dengue,” Reuters, January 21, 2007, sec. Healthcare & Pharma, https://www.reuters.com/article/us-malaria-singapore-s-idUSSP28481920061211; World Health Organization, “Dengue Haemorrhagic Fever Control Programme in Singapore: A Case Study on the Successful Control of Aedes aegypti and Aedes albopictus Using Mainly Environmental Measures as a Part of Integrated Vector Control.” ↩

  15. "VIRUS CARRIER TRACKED,” The Straits Times, January 7, 1961. ↩

  16. Kaiwei Ho et al., “Epidemiology and Control of Chikungunya Fever in Singapore,” Journal of Infection 62, no. 4 (April 1, 2011): 263–70, https://doi.org/10.1016/j.jinf.2011.02.001. ↩

  17. Zheng Jie Marc Ho et al., “Outbreak of Zika Virus Infection in Singapore: An Epidemiological, Entomological, Virological, and Clinical Analysis,” The Lancet Infectious Diseases 17, no. 8 (August 1, 2017): 813–21, https://doi.org/10.1016/S1473-3099(17)30249-9. ↩

  18. Kai Lok Chan, “Control of Aedes Vectors of Dengue Fever/Dengue Haemorrhagic Fever in Singapore,” Japanese Journal of Tropical Medicine and Hygiene 16, no. 2 (1988): 113–20. ↩

  19. World Health Organization, “Dengue Haemorrhagic Fever Control Programme in Singapore: A Case Study on the Successful Control of Aedes aegypti and Aedes qlbopictus Using Mainly Environmental Measures as a Part of Integrated Vector Control.” ↩

  20. World Health Organization. ↩

  21. Benjamin A. McKenzie, Alan E. Wilson, and Sarah Zohdy, “Aedes albopictus Is a Competent Vector of Zika Virus: A Meta-Analysism, PLOS ONE” 145, no. 5 (2019): e0216794, https://doi.org/10.1371/journal.pone.0216794; Donald A. Yee et al., “Robust Network Stability of Mosquitoes and Human Pathogens of Medical Importance,” Parasites & Vectors 15, no. 1 (June 20, 2022): 216, https://doi.org/10.1186/s13071-022-05333-4. ↩

  22. N. G. Gratz, “Critical Review of the Vector Status of Aedes albopictus,” Medical and Veterinary Entomology 18, no. 3 (2004): 215–27, https://doi.org/10.1111/j.0269-283X.2004.00513.x. ↩

  23. Hapuarachchige Chanditha Hapuarachchi et al., “Epidemic Resurgence of Dengue Fever in Singapore in 2013-2014: A Virological and Entomological Perspective,” BMC Infectious Diseases 16, no. 1 (June 17, 2016): 300, https://doi.org/10.1186/s12879-016-1606-z. ↩

  24. Zhaki Abdullah, “Singapore Records 19 Dengue Deaths in 2022, Nearly Four Times 2021’s Toll,” The Straits Times, February 1, 2023, https://www.straitstimes.com/singapore/health/singapore-records-19-dengue-deaths-in-2022-nearly-four-times-2021-s-toll. ↩

  25. Kai Lok Chan, “Control of Dengue Vectors,” Asian Journal of Infectious Diseases 2 (1978): 139–45. ↩

  26. Chan; National Environment Agency, “Do the Mozzie Wipeout Campaign,” accessed April 5, 2023, https://www.nea.gov.sg/programmes-grants/campaigns/do-the-mozzie-wipeout-campaign. ↩

  27. “‘Fish Fighters’ Go Underground,” New Nation, October 18, 1973. ↩

  28. World Health Organization, “Dengue Haemorrhagic Fever Control Programme in Singapore: A Case Study on the Successful Control of Aedes aegypti and Aedes albopictus Using Mainly Environmental Measures as a Part of Integrated Vector Control.” ↩

  29. Ministry of Health, “Destruction of Disease-Bearing Insects Act, 1968,” September 23, 1968, https://www.nas.gov.sg/archivesonline/data/pdfdoc/PressR19680923.pdf. ↩